Iontophoresis involves the application of an electric current to introduce ions of soluable salts into tissues of the body for the therapeutic purposes. This technique may be advantageously utilized in those cases where atopical application of such substances does not result in sufficient penetration into body tissue to accomplish satisfactory effects or where tissue penetration is otherwise undesirable. Application of the electric current causes migration of the ions into the tissue to greater depths, such migration being in proportion to the duration of current application and current density.
Iontophoresis devices have been developed for dental treatments in applying medications, anesthetizing agents, and desensitizing ions such as strontium and flouride. Essentially these devices consist of a battery, an applicator electrode carrying the ionic substance, and simple circuitry operable to provide a small current into the tissue to be treated. Typically, the circuit is closed by a grounding contact with the patients hand or other body surface. Generally, previous iontophoresis devices have operated as constant voltage devices, with varying current depending upon the impedance of the treated tissue.
Although a number of different embodiments of the above illustrated devices have been developed, utility has been limited to tissue such as that of the oral environment due to its relatively low impedance. Because of the more compatible electrical characteristics of this tissue, the iontophoresis device will effectively operate at low voltage. If the treatment area consists of epidermal tissue, however, the simple circuitry previously considered sufficient for oral applications will not be operative since the greater impedance within the conducting path requires a higher voltage to maintain effective current. Such higher voltage greatly increases the risk of shocks or burns to the patient and has discouraged a general application of iontophoresis treatments.
Inasmuch as the extent of ion migration is a function of the current flow, the rate of iontophoresis increases with a corresponding increase in current. Limitations as to the amount of current are based primarily on considerations of patient safety. If the current density becomes too high, the local resistance of the tissue results in burns or other electrical damage. Where tissue is burned, the resulting decrease in resistance increases current flow, compounding the danger of serious burns to the tissue.
A second limiting factor involves the discomfort associated with voltage applied to human tissue. Although current is the damaging force which causes burns and other physical disturbance, voltage is the primary cause of the alarming sensations of pain and shock. Voltages in excess of 40v will customarily become perceptable. The actual physical damage will depend upon the impedance of the tissue since the combined values of voltage and impedance define the amount of local current density. The high resistance of epidermal tissue frequently requires voltage capability which could easily exceed the 40v threshold and thereby result in pain to the patient. Since the primary object of this iontophoresis device is to provide a nonpainful method of anesthetizing a patient, any patient discomfort due to high voltage will defeat this purpose.
Voltage can be minimized by maintaining the treated tissue at the lowest possible resistance. By utilizing appropriate techniques, epidermal impedance can be lowered to approximately 10K ohms to 40K ohms, depending upon the patient. Assuming a 15K ohms resistance across the electrodes and a 2 ma current, the required conduction voltage is 30v. Such a current level could be effectively used for iontophoresis on most patients without causing patient discomfort.
To minimize treatment time, an iontophoresis device should operate at the maximum current possible without causing burns or shocks, since lower current flow requires longer periods of application to reach the same degree of ion penetration. A constant voltage device is not suitable to provide a constant current to the variable higher impedance of epidermal tissue and the accompanying conductive gel. Although a constant current device will accomplish this purpose, additional circuitry for holding the maximum current through the electrodes is necessary to achieve optimal safe performance while at the same time protecting the patient from the aforementioned adverse effects of current regulation. Maintenance of the constant current results in proportional increases in voltage with an increase in impedance across the electrodes in accordance with Ohm's Law. Herein lies one of the major hazards of higher voltage, current regulated iontophoresis devices.
Assuming that the device is current regulated and operating at 3 ma in contact with the patient's skin, then if the device is suddenly removed a short distance from the skin, the increased impedance resulting from the additional air gap separating the electrodes develops an extreme surge in voltage as the circuitry attempts to maintain the 3 ma current. To suddenly return the electrode toward the skin might result in an electrical jolt due to the high voltage across the decreasing impedance. Such current surges result in shocks, burns and similar alarming and dangerous effects.
Consequently, the present application of iontophoresis treatment through epidermal tissue is generally limited to the pilocarpine test for cystic fibrosis in which pilocarpine is conducted into the patient to induce sweating. Unfortunately, the appreciable electric current required has resulted in the frequent occurrence of the aforementioned dangers. The incidence of burns is significant. Often, current flow reaches ranges capable of causing muscular spasms or ventricular fibrillation, particularly in small children.
The recurrence of such effects has been a major factor in discouraging the application of iontophoresis techniques to potential uses other than in the dental care area. The broad spectrum of utility of iontophoresis, however, suggests the need for a safe method which could provide painless local anesthesia and improved methods of asceptization for general medical use. Such a method could be applied prior to insertion of any needle, particularly large cannulae which are both painful and unnerving to patients. Small children and infants would be less fearful and more cooperative, thereby making the treatment procedures less difficult.
Patients on hemodialysis and hospital patients who often require regular insertion of large needles are frequently prepared by injection of a small amount of xylocaine to anesthetize the area prior to insertion. In addition to the pain and psychological effect of this initial injection, there are accompanying traumatizing effects to the skin. All of these adverse effects could be minimized with a safe method of epidermal iontophoresis.
As a corrolary to the aforementioned problems with injections, the removal of warts and treatment of boils, absesses and other infections which cause swelling and tensing of tissue create special difficulties. Desensitization of such sensitive tissue is frequently as painful as the nonanesthetical surgury and therefore defeats one of the primary purposes of the procedure. In addition to the pressure of the needle insertion, the high concentration of anesthetic at the opening of the cannula may result in a burning sensation until the drug difuses into surrounding tissue. Frequently the injected fluid forms a temporary pocket which deforms the tissue and increases pressure on the surrounding nerves. Such disfiguration resulting from fluid pockets is particularly troublesome in plastic surgery where the tissue must be in its natural condition to ensure proper reformation. Virtually any treatment involving conduction of medicament through the epidermis could be accomplished by iontophoresis techniques.